Cellulose membranes are well known in the art and are used, e.g., in dialysis, reverse osmosis, and ultrafiltration. One important application for membranes of this type is dialyzers, particularly for hemodialysis.
Extremely demanding biocompatibility requirements are placed on dialysis membranes for hemodialysis so that the blood flowing past the membranes is affected as little as possible and the side effects for the dialysis patient are minimized. The membrane material used is crucial with respect to biocompatibility among other factors.
To prepare dialysis membranes, the cellulose is either regenerated by saponification of cellulose acetate or by precipitation with acids, lyes, saline solutions, or water from cuoxam (i.e., copper-ammonium complex, Schweitzer's reagent) solutions or viscose solutions or precipitated from solutions in aprotic solvents with salts and from solutions in tertiary amine oxide.
The starting materials for making cellulose membranes are cotton fibers, in particular, cotton linters. In addition to cellulose, cotton as a natural product, contains a number of additional substances such as pectins, waxes, proteins, mineral components and other, especially organic, components as natural products. Depending on their freedom from contamination, seed hull residues are also present.
Cotton for textile applications consists of relatively long fibers, and the first step in its production is to separate the cotton fibers from the cotton seeds (i.e., ginning). After ginning, short hairs still remain on the hulls of the mature cotton seed capsules, which short hairs are called cotton linters or linters. The cotton linters are separated mechanically from the bolls in further process steps and, by comparison to textile raw cotton, usually contain a higher proportion of contaminants such as seed hull residues.
Linters have the following approximate composition:
______________________________________ Linters-cellulose: approx. 75 to 90 wt. % Water: approx. 6 to 8 wt. % Oils, fats, waxes: approx. 1 to 3 wt. % Seed hull residues: approx. 1 to 5 wt. % Other contaminants: approx. 1 to 5 wt. % ______________________________________
The variations depend, in particular, on the original quality and the way in which the linters were obtained. By contrast with other natural fibers, e.g., jute, flax, sisal and wood, linters contain practically no lignin. Only the seed hulls themselves contain xyloses and lignin. Depending on the type and origin of the cotton plants, linters-cellulose has an average degree of polymerization (DP) of 3,000 to 8,000.
Before further processing into membranes, the cotton and, to a greater degree, the cotton linters, must undergo treatment to clean and digest the fibers. The treated cotton fibers or linters when dry should have a cellulose content of at least 98 wt. %. Moreover, the smallest possible content of extractable mono sugars and, in particular, oligo glucanes, is necessary with a view to good biocompatibility properties. As a rule, a treatment that suffices for these requirements is kier boiling followed by bleaching.
The purpose of kier boiling is to remove or break down the above-listed additional substances and contaminants, particularly the seed hull residues, namely the non-cellulose components. This is usually done in an alkali medium (i.e., an alkaline liquor) in which the cotton or linters are digested, hydrophilized, swollen, and become receptive to bleaches and dyes, for example. Often with kier boiling, the desired degree of polymerization of the cellulose is deliberately set at the same time in order, for example, to reduce the viscosity of the cellulose solutions made therefrom for further processing.
According to the prior art, careful attention must be paid to see that no oxygen, e.g., atmospheric oxygen, is added during kier boiling. Before the start of kier boiling, all the atmospheric oxygen must be removed from the boiling vessels. Failing to preliminarily remove oxygen in prior art processes results in oxidatively damaged fibers, such as cellulose that is damaged in an uncontrolled fashion, in addition to which the degree of polymerization and the yield of pretreated linters are undesirably reduced.
Following kier boiling, the cotton fibers or cotton linters are subjected to additional kier boiling in a neutral or slightly alkaline medium and rinsed thoroughly with water, and possibly, the last alkali residues are acidified. After kier boiling, the color of the cotton fibers or linters is slightly yellowish to gray.
To break down the dyes that remain in the cellulose and for complete removal of the contaminants that have already been largely removed or loosened by kier boiling, the cotton or linters are then subjected to bleaching. In this step, the chemical and physical properties of the cellulose should be retained.
Both kier boiling and bleaching can be single-step or multi-step processes. Since according to the prior art they are carried out with different chemicals and reagents and under different reaction conditions, as a rule, kier boiling and bleaching are two separate process steps, carried out sequentially and separately, each of which is connected with one or more washing steps.
According to previous methods, large quantities of water, energy, and chemicals per kilogram of cotton fiber or linters are sometimes necessary. For example, in the typical kier boiling and bleaching process in industrial use, a total of 130 to 170 liters of water per kilogram of cotton linters is consumed, taking approximately 20 hours. In this classical method, the total organic waste is typically approximately 200 g/kg cotton linters and the inorganic waste from the lye used in excess for neutralization is approximately 200 to 300 g/kg cotton linters (calculated as Na.sub.2 SO.sub.4).
The literature also teaches intermediate steps for cotton fibers and textile cotton goods. For example, P. Heermann (Enzyklop adie der textil-chemischen Technologie, Verlag Julius Springer, Berlin 1930) describes how the addition of oxidants such as sodium peroxide, perborate, or Aktivin to the kier boiling liquor results not only in kier boiling itself but also in a certain pre-bleaching of the cotton. Also, by means of the oxidation bleaching method, purification of the cotton is achieved in addition to bleaching. Here, the purification is characterized as being different from the one linked to kier boiling; in particular, reduced absorptivity or hydrophilicity of the cotton goods so treated is observed. Recently, a peroxide bleaching method has been reported (D. Bassing, Textilveredelung (22) 6, 1987, pp. 226-230) in which, in a one-step process without prior bleaching or boiling-off steps and with complexing agents being added, good absorptivity is achieved as well. These references do not state, however, whether the reported method can also be used with satisfactory results to treat cotton linters. The use of the cotton fibers treated by this method as a raw material for membrane manufacture is also not mentioned.
DE-OS 19 54 267 describes treatment of cellulose-containing materials with alkali, introducing air or gaseous oxygen, in which the process is carried out in the presence of a magnesium salt and a complexing agent. This method is used, in particular, to prepare wood cellulose containing lignin as it has particular advantages in removing the lignin. With lignin-free celluloses, the complexing agents added serve to prevent uncontrolled breakdown of the cellulose molecules. The oxygen treatment method described in the cited reference is, however, only one of several treatment stages and always relates to the manufacture of pulps, preferably lignocellulose, namely starting materials for the paper industry, and thus not to manufacture of starting materials for membranes.
The same applies to German Patent 21 41 757 in which the use of oxygen-containing gas is also described and reference is made exclusively to the bleaching of cellulose pulp previously digested by chemical means and bleaching lye and digesting lye from the previous treatment step must be simultaneously present in the liquor to achieve the desired bleaching effect and an improved delignification effect. In this case too, the oxygen bleaching step represents only one process step out of several treatment and bleaching steps for preparation of cellulose pulp.
Each of the prior art processes referred to also generate considerable quantities of wastewater, and large quantities of water, energy, and chemicals are needed to run the process.
For manufacturing cellulose membranes, cotton fibers or cotton linters are brought into the dissolved form after the kier boiling/bleaching treatment, namely after two different, independent process steps, usually followed by drying. The solutions are formed into hollow filaments, tubes, or films, with the cellulose being either regenerated or precipitated.
When kier-boiled, bleached linters of the prior art are used, the dialysis membranes made from regenerated cellulose exhibit distinct complement activation in hemodialysis. To improve biocompatibility by, for instance, reducing complement activation while retaining other favorable properties of the cellulose membranes, there are various methods such as cellulose modified by substitution, admixture of cellulose modified by substitution or admixture of other suitable polymers. Such methods are described in documents DE-OS 35 24 596, EP 0 416 377, and DE-OS 40 17 745. However, such methods always increase expenditures by adding necessary process steps.